This invention relates to a novel method for electrically processing completely-assembled cathode-ray tubes having electron guns with indirectly-heated oxide cathodes. One or more guns may be installed in each cathode-ray tube.
In the manufacture of cathode-ray tubes having electron guns with indirectly-heated oxide cathodes, it is the practice to process the tubes after they have been completely assembled, exhausted of gases and sealed, so that the tubes become operative. The tubes' operations are stabilized and the operating lives are lengthened. For this processing, each gun in the tube is usually subjected is succession to the steps of "spot knocking," "hot shot," "high-voltage aging" and "low-voltage aging."
In one form of the "spot-knocking" step, the cathode, the heater and the low-voltage electrodes G1, G2 and G3 are grounded, and a pulsed positive voltage which peaks at about 200% of the normal ultor voltage is applied to the high-voltage electrode G4 and to the anode (the internal conductive funnel coating) of the tube for about 2 minutes to burn off loose particles which may reside between the electrodes in the gun.
In one form of the hot-shot step, the cathode is activated by heating it to an abnormally high temperature, as by applying about 10 to 12 volts across the cathode heater (where 6 to 7 volts are normally applied) for about 2 minutes, with all of the electrodes and the anode floating electrically. The initial portion of the hot-shot step may also be used to convert the cathode coating from carbonates to oxides. Converting the cathode coatings is usually done when the tubes are being exhausted of gases prior to sealing.
In one form of the high-voltage aging step, which usually lasts for about 3 to 60 minutes, the cathode is emitting, various combinations of constant voltages including ground potential are applied to the G1, G2 and G3 electrodes, and a high voltage, substantially higher than normal operating ultor voltage, is applied to the high-voltage electrode G4. The high-voltage aging step allows time-related defects to manifest themselves and, in most cases, cure themselves. The high-voltage aging step is optional and is omitted from the processing of many tubes.
In one form of the low-voltage aging step, sometimes called the cathode-aging step, which usually lasts for about 30 to 90 minutes, the cathode is emitting, various combinations of constant positive voltages are applied to the control electrode G1, the screen electrode G2 and the focus electrode G3, and the high-voltage-electrode G4 is floating electrically. The low-voltage aging step permits the emission from the cathode to stabilize and the various electrodes to outgas due to bombardment by electrons from the cathode. Although low-voltage aging achieves these objects in most tubes, nevertheless, after low-voltage aging a significant portion of tubes exhibit (a) too low an initial cathode-emission level and/or (b) a drop of slump in cathode emission level from its initial level after only a short period of use, or (c) when, after a holding period or shelf life, a portion of the tubes exhibit the characteristics noted in (a) and (b). The conditions (a), (b) and (c) are more prone of tubes whose cathode coatings are converted during the hot-shot step instead of during the period when the envelope was exhausted of gases prior to sealing.